This invention relates to the domain of methods for determining the orientation of objects in space.
More precisely, it relates to a method of assigning weighting coefficients to measurements of a succession of stars acquired by a star sensor in order to determine a spatial orientation.
An increasing number of artificial satellites are orbiting around the Earth or are launched into space. Obviously, it is important to know the position of satellites in space, but it is also essential to know their attitude, in other words their orientation with respect to an inertial coordinate system in the celestial dome.
A star sensor is an instrument that supplies its own attitude measurement calculated from direction measurements of stars that it carries out, or that directly transmits the list of measurements of elementary directions to a client device, for example such as the central software on a satellite on which the sensor is located.
FIG. 1 shows a satellite 1, for example in an orbit 2 around a planet 3, for example the Earth, at an altitude 13 above the planet surface. The satellite 1 comprises a star sensor 4 on one of its walls facing the stars in the celestial dome, and with a view field diagrammatically shown by the trunk of the cone 5. The circle 8 represents the part of the celestial dome observable in the view field 5. The stars 10 and 11 diagrammatically represent some of the stars that can be observed by the sensor 4. It can be understood that a plurality or a large number of stars can be observed at the same time within the circle 8. The stars 9 and 12 represent stars in the celestial dome outside the view field of the sensor.
The satellite 1 displaces in the orbit 2 along the arrow 6. Consequently, it can be understood that the circle 8 also displaces on the celestial dome along the arrow 7. Thus, the star 9 used to be in the circle 8, and the star 12 will be in the circle 8 in the future of the satellite displacement.
Most frequently, the attitude of the sensor 4 or the satellite 1 is calculated using only some of the stars present in the view field 8 of the star sensor 4. For example, about ten stars may be chosen to make an attitude calculation. The direction of the star 10 or 11 with respect to a coordinate system 15 related to the sensor 4 is measured in the attitude calculation. Since the direction of the star 10 or 11 with respect to a known inertial coordinate system 14 is also known, the direction of the coordinate system 15 (and therefore of the satellite 1 or the sensor 4) with respect to the coordinate system 14 of the celestial dome may be deduced.
The precision of the attitude estimate depends on the choice of stars to be measured for the direction calculation.
The choice of stars is also important when star direction measurements are transmitted directly and are used by the client device of the star sensor.
In some cases, the stars may be associated with a weight instead of being selected or rejected, which weights the importance to be assigned to them in subsequent processing.
In other cases, the star sensor itself makes a first selection of stars, makes measurements on these selected stars and may send them accompanied by a weight or transformed into a global attitude estimate.
The measurements may also originate from different star sensors onboard the same satellite.
In all cases, a selection is a set of potential or previously made measurements of star directions, for which the validity date is identical. As we have already said, weights may be associated with each measurement. Over time, a star sensor 4 generates a succession of selections, measurements being made on all or some of the selected stars.
Traditionally, stars chosen as being the best stars will be selected for as long as possible. In the early dates of stars sighting, it was difficult enough simply to lock onto a star. Therefore, once it was found it was not released.
One of the criteria for choosing stars in a selection is particularly the magnitude of each star that can be observed in the view field. Thus, the brightest stars should be chosen, namely stars with the lowest magnitude. One of the other selection criteria is the distance of the star from the optical axis of the view field. More precisely, FIG. 2 shows the circle 8 moving along the arrow 7 across the celestial dome. Choosing stars remote from the optical centre 83 and located within a reference area 81 within the circle 8 can improve the precision of the estimate depending on the optical axis 83 of the sensor. The axis 83 is the most likely to be affected by measurement errors. Stars located in an area reference 82 will be close to the optical axis 83 at one time or another and will provoke measurement errors. A star that is too close to the optical centre 83 will be rejected and another star further from the optical centre 83 will then be selected.
At the moment, selection methods give excessive importance to these two criteria, with the result that stars are not changed frequently.
However, the above methods have disadvantages.
Some stars, for which the direction is particularly badly estimated by sensor 4, disturb the global attitude estimate throughout the duration in which they are selected. This duration may be fairly long when the attitude of the star sensor 4 varies only slightly and when its view field is fairly wide. FIG. 3 diagrammatically shows such a situation. FIG. 3 shows the error on the attitude of the sensor 4 as a function of time. Changes 30 in the error level of the graph are due to star selection changes for determination of the attitude. It is seen that errors due to the choice of a selection are relatively long compared with the oscillations 31 on each plateau of the curve, which are due to observation errors of each star in the selection. It is observed that the errors 31 cancel out due to averaging over a relatively short time compared with the time during which each selection is observed. In other words, the noise due to oscillations 31 can easily be filtered by existing techniques for processing of data received from star sensors by client devices, since it is within the high frequencies of the frequency spectrum of the signal acquired by the sensor. The fact that the selection changes only infrequently results in low frequency noise that is difficult to filter.
More generally, methods of assigning a weighting coefficient according to prior art are incapable of controlling the noise dispersion with time due to each selection.
Furthermore, some phenomena (for example distortion) vary depending on the position of stars in each selection in the view field.
More generally, methods for assigning weighting coefficients according to prior art are incapable of controlling these space-time dispersion phenomena for stars to which high weights are assigned.